The use of optical fiber communication systems has increased significantly during the last few years. It appears likely that the use of this mode of communications will continue to increase in the future. Companies engaged in the manufacture of components for these systems continue to seek ways to reduce the cost thereof.
Presently, optical fibers are being manufactured in processes which include the reaction of a silicon-containing gas and the reaction of a germanium-containing gas to form a deposited glass core having suitable optical properties. These processes are used to fabricate preforms which is the first step in making lightguide fibers. One such process which is known as a modified chemical vapor deposition (MCVD) process is described in J. B. MacChesney, "Materials and Processes for Preform Fabrications-Modified Chemical Vapor Deposition," Vol. 68, Proceedings of IEEE, pp. 1181-1184 (1980).
The input to the MCVD process comprises oxygen as a carrier gas and reactant vapors such as germanium tetrachloride (GeCl.sub.4), silicon tetrachloride (SiCl.sub.4) and phosphorous oxychloride (POCl.sub.3). These reactant vapors are passed through a glass tube which is heated to a temperature in the range of 1600.degree. to 1800.degree. C. by an oxyhydrogen torch. The effluents from the induced germanium and silicon reactions typically include particulates as well as gaseous materials.
This process is relatively inefficient in its incorporation of germanium into the deposited core. Further, in processes such as MCVD which are performed in the substantial absence of hydrogen, such as the hydrogen present in water, germanium in the effluent is not found primarily in the particulates. Instead, the gaseous portion of the effluent contains the major portion of the unreacted germanium. It has been found that about 70% of the germanium tetrachloride does not react and is moved out as a vapor, and 30% of the germanium tetrachloride is converted to germanium dioxide (GeO.sub.2) within the substrate tube of which about 50% is deposited. In other words, about 15% of the original amount of the germanium tetrachloride is deposited as particulate matter in the preform whereas about 15% leaves the tube as undeposited particulates. As a result, relatively large quantities of unused germanium, which is the most expensive raw material used in the manufacture of lightguide fibers, are rejected. Moved along with the germanium tetrachloride vapor are solids comprising silicon dioxide (SiO.sub.2), GeO.sub.2 and phosphorous pentoxide (P.sub.2 O.sub.5), and chlorine gas. By hydrolysing the germanium in these gases and collecting it in a form suitable for recylcing, a substantial reduction in the cost involved in the manufacture of optical fibers is achieved.
One technique for removing the germanium from the vapor phase, and any germaniumin particulate form, involves a process in which a liquid medium is recirculated in a loop. The manufacturing effluent from lathes which are used in the production of optical preforms is scrubbed with an aqueous medium to ensure the hydrolysis of germanium-containing gases such as GeCl.sub.4. The aqueous medium is filtered to remove particulates and recycled to treat subsequent process effluent and to be refiltered. The concentration of germanium in the medium is substantially increased by the recycling process and by the dissolution of germanium-containing particulates. Portions of the recirculating medium are periodically or continuously removed and treated to precipitate germanium which is then separated from the remaining liquid by conventional means. See U.S. Pat. No. 4,385,915 which issued on May 31, 1983 in the names of J. A. Amelse et al.
There are problems associated with the recirculating process for recovering germanium. For the economical recovery of germanium, it becomes necessary to maintain a particular concentration level of germanium such as, for example, greater than 600 ppm in the recovered constituent which is in the form of a filter cake. Should the concentration decrease, it becomes more expensive to recover the germanium from the filter cake. Maintaining a particular concentration level becomes a problem because the germanium input to the scrubbers varies as a function of the type and quantity of product manufactured. Each time the recirculating liquid medium is moved through the filter of the above-described process, the germanium concentration is increased. The filter may be controlled to adjust the amount of the recirculating liquid medium which passes through the filter compared to that which is filtered out. As the recirculation rate is increased, the germanium concentration increases, but at the same time the level of particulates in the recirculating liquid increases. As their number increases, the particulates agglomerate which could cause clogging of the filter. Further, a system malfunction, such as pH drop in the recirculating liquid, for example, if left unattended could cause the precipitation of solids in the loop which would cause the filter to clog, resulting in a shutdown of the operation.
Solutions to the foregoing problems have not been provided by the prior art. What is still needed in order to improve the recovery of germanium and thereby reduce costs is a filtration system that is capable of maintaining a required germanium concentration and of controlling the particulates in the recirculating liquid. This must be accomplished notwithstanding the amount of the germanium which is flowed into the loop. Also, the loop should be operable during some non-equilibrium conditions such as during start up of the process.